Currently, development of optical networks (especially core networks) for further increasing the transmission rate and the bandwidth is proceeding. In some cases, the optical networks using wavelength division multiplexing (WDM) are used. In the optical networks, chromatic dispersion (hereinafter, referred to simply as dispersion) can cause waveshape distortion during propagation of optical signals in optical fibers. The dispersion is a type of dispersion caused by the variations with the wavelength in the propagation speed of the optical signal through the optical fiber. The amount of dispersion varies with the length, material, and the like of the optical fiber through which the optical signal propagates.
The dispersion can cause bit errors when a destination node extracts information from an optical signal. The bit errors tend to increase with the total amount of dispersion accumulated until the optical signal reaches the destination (end) node. Therefore, in some optical networks, a dispersion compensation module (DCM) is arranged at the end point of the optical fiber or the like. The dispersion compensation module is an optical device which has an inverse characteristic to the dispersion characteristic of the optical fiber. For example, an optical fiber having the inverse characteristic can be used as the dispersion compensation module. The total amount of dispersion can be reduced when the optical signal passes through the dispersion compensation module.
Therefore, in many cases of designing an optical network, a bit error rate which is tolerable in the destination node (e.g., 10−15) is preset. In such cases, it is preferable to design the optical network in consideration of the tolerable bit error rate so that, whatever path the optical signal passes through, the total amount of dispersion at the destination node falls within a certain range. On the other hand, according to a method for designing dispersion compensation which is disclosed, for example, in the International Patent Publication No. WO2005/006604, the amounts of dispersion in dispersion compensation modules arranged in each path are set so that the total amount of dispersion in every path falls within a tolerance range which is preset as a transmittability condition.
According to a method for selecting an optimal path from among a plurality of paths which is disclosed, for example, in the International Patent Publication No. WO2005/032076, information on the transmission characteristics is accumulatively transmitted from a start node to a destination node, and the optimal path is determined on the basis of the accumulative information on the transmission characteristics received by the destination node. Specifically, a path for use in transmission of optical signals is selected from among a plurality of paths each extending from the start node to the destination node, in consideration of dispersion. However, according to the method disclosed in WO2005/032076, only the accumulative information on the transmission characteristics received by the destination node is considered. Therefore, there is room for improvement in transmission quality by suppressing distortion in the optical signal for the following reason.
In some optical networks, the optical signals are amplified by optical amplifiers at intermediate nodes. The optical amplifiers have nonlinear characteristics. Therefore, when an optical signal affected by a great amount of accumulated dispersion is amplified, a nonlinear component remains even after the optical signal passes through the dispersion compensation module, i.e., it is impossible to sufficiently compensate for the distortion. On the other hand, even when the amount of accumulated dispersion is extremely close to zero, nonlinear distortion occurs in the optical signal. This is because a strong nonlinear effect occurs when an optical signal the power of which is excessively focused is inputted from an optical amplifier into an optical fiber. Therefore, it is preferable to select a path in consideration of the condition of the optical signal in the entire path.